Rhizobial counts in peat inoculants vary amongst legume inoculant groups at manufacture and with storage: implications for quality standards

Background and aims

Inoculation of legumes at sowing with rhizobia has arguably been one of the most cost-effective practices in modern agriculture. Critical aspects of inoculant quality are rhizobial counts at manufacture/registration and shelf (product) life.

Methods

In order to re-evaluate the Australian standards for peat-based inoculants, we assessed numbers of rhizobia (rhizobial counts) and presence of contaminants in 1,234 individual packets of peat–based inoculants from 13 different inoculant groups that were either freshly manufactured or had been stored at 4 °C for up to 38 months to determine (a) rates of decline of rhizobial populations, and (b) effects of presence of contaminants on rhizobial populations. We also assessed effects of inoculant age on survival of the rhizobia during and immediately after inoculation of polyethylene beads.

Results

Rhizobial populations in the peat inoculants at manufacture and decline rates varied substantially amongst the 13 inoculant groups. The most stable were Sinorhizobium, Bradyrhizobium and Mesorhizobium with Rhizobium, particularly R. leguminosarum bv. trifolii the least stable. The presence of contaminants at the 10^?6 level of dilution, i.e. >log 6.7 g^?1 peat, reduced rhizobial numbers in the stored inoculants by an average of 37 %. Survival on beads following inoculation improved 2–3 fold with increasing age of inoculant.

Conclusions

We concluded that the Australian standards for peat-based rhizobial inoculants should be reassessed to account for the large differences amongst the groups in counts at manufacture and survival rates during storage. Key recommendations are to increase expiry counts from log 8.0 to log 8.7 rhizobia g^?1 peat and to have four levels of inoculant shelf life ranging from 12 months to 3 years.     

Utilization of net photosynthate for nitrogen fixation and protein production in an annual legume

The economy of C and N in nodulated cowpea (Vigna unguiculata [L.] Walp.) was described in terms of fixation of CO₂ and N₂, respiratory losses of C, and the production of dry matter and protein.     

Mutations in CSPP1, Encoding a Core Centrosomal Protein,Cause a Range of Ciliopathy Phenotypes in Humans

Ciliopathies are characterized by a pattern of multisystem involvement that is consistent with the developmental role of the primary cilium. Within this biological module, mutations in genes that encode components of the cilium and its anchoring structure, the basal body, are the major contributors to both disease causality and modification. However, despite rapid advances in this field, the majority of the genes that drive ciliopathies and the mechanisms that govern the pronounced phenotypic variability of this group of disorders remain poorly understood. Here, we show that mutations in CSPP1, which encodes a core centrosomal protein, are disease causing on the basis of the independent identification of two homozygous truncating mutations in three consanguineous families (one Arab and two Hutterite) affected by variable ciliopathy phenotypes ranging from Joubert syndrome to the more severe Meckel-Gruber syndrome with perinatal lethality and occipital encephalocele. Consistent with the recently described role of CSPP1 in ciliogenesis, we show that mutant fibroblasts from one affected individual have severely impaired ciliogenesis with concomitant defects in sonic hedgehog (SHH) signaling. Our results expand the list of centrosomal proteins implicated in human ciliopathies.     

The contributions of nitrogen-fixing crop legumes to the productivity of agricultural systems

Data collated from around the world indicate that, for every tonne of shoot dry matter produced by crop legumes, the symbiotic relationship with rhizobia is responsible for fixing, on average on a whole plant basis (shoots and nodulated roots), the equivalent of 30–40 kg of nitrogen (N). Consequently, factors that directly influence legume growth (e.g. water and nutrient availability, disease incidence and pests) tend to be the main determinants of the amounts of N₂ fixed. However, practices that either limit the presence of effective rhizobia in the soil (no inoculation, poor inoculant quality), increase soil concentrations of nitrate (excessive tillage, extended fallows, fertilizer N), or enhance competition for soil mineral N (intercropping legumes with cereals) can also be critical. Much of the N₂ fixed by the legume is usually removed at harvest in high-protein seed so that the net residual contributions of fixed N to agricultural soils after the harvest of legumegrain may be relatively small.Nonetheless, the inclusion of legumes in a cropping sequence generally improves the productivity of following crops. Whilesome of these rotational effects may be associated with improvements in availability of N in soils, factors unrelated to N also play an important role. Recent results suggest that one such non-N benefit may be due to the impact on soil biology of hydrogenemitted from nodules as a by-product of N₂, fixation.     

Competition for access to mates predicts female-specific ornamentation and male investment in relative testis size

Sexually selected ornaments are highly variable and the factors that drive variation in ornament expression are not always clear. Rare instances of female-specific ornament evolution (such as in some dance fly species) are particularly puzzling. While some evidence suggests that such rare instances represent straightforward reversals of sexual selection intensity, the distinct nature of trade-offs between ornaments and offspring pose special constraints in females. To examine whether competition for access to mates generally favors heightened ornament expression, we built a phylogeny and conducted a comparative analysis of Empidinae dance fly taxa that display female-specific ornaments. We show that species with more female-biased operational sex ratios in lek-like mating swarms have greater female ornamentation, and in taxa with more ornate females, male relative testis investment is increased. These findings support the hypothesis that ornament diversity in dance flies depends on female receptivity to mates, which is associated with contests for nutritious nuptial gifts provided by males. Moreover, our results suggest that increases in female receptivity lead to higher levels of sperm competition among males. The incidence of both heightened premating sexual selection on females and postmating selection on males contradicts assertions that sex roles are straightforwardly reversed in dance flies.     

Quantifying below-ground nitrogen of legumes. 2. A comparison of 15N and non isotopic methods

Accurate information on below-ground nitrogen (N) of legumes is necessary for quantifying legume effects on soil N pools and on the N economies of crops following legumes in rotation systems. We report a series of glasshouse pot experiments to determine below-ground N (BGN) of the four legumes, fababean (Vicia faba), chickpea (Cicer arietinum), mungbean (Vigna radiata) and pigeonpea (Cajanus cajan) using both ¹⁵N shoot-labelling and ¹⁵N-labelled soil isotope-dilution methods, a mass N balance approach and the physical recovery of nodulated roots. Data from the ¹⁵N shoot-labelling experiment were manipulated in different ways in an attempt to counter errors associated with uneven ¹⁵N enrichment of roots and nodules. Values for BGN as percent of total plant N based on the physical recovery of nodulated roots ranged from 4 to 15%. With ¹⁵N shoot-labelling, a total of 8.11 mg ¹⁵N was supplied to each pot (six plants) as 0.5% ¹⁵N urea using either leaf-flap (fababean, mungbean and pigeonpea), petiole (chickpea) or leaf-tip (wheat) feeding. Calculations based on measurement of ¹⁵N enrichments of harvested plant parts and root-zone soil suggested that BGN represented 39% of total plant N for fababean, 53% for chickpea, 20% for mungbean and 47% for pigeonpea. The value for wheat was 60%. Adjustment for uneven nodulation patterns on the roots and nodule ¹⁵N depletion, resulting in different ¹⁵N enrichments between nodulated and unnodulated roots, reduced the fababean value to 37% and chickpea to 42%. Values using the other methods were generally in the same range, viz. 15–57% (simple ¹⁵N balance), 11–52% (soil ¹⁵N dilution) and 30–52% (mass N balance). We conclude that physical recovery of roots was the most inaccurate method for estimating BGN. Average values for BGN as percent of total plant N using all isotopic and mass N balance methods were 30% for fababean, 48% for chickpea, 28% for mungbean, and 43% for pigeonpea.¹⁵N shoot-labelling may be the best method for quantifying BGN of field-grown plants. The methodology is simple, apparently accurate provided care is taken in obtaining representative nodulated root samples and, unlike the soil ¹⁵N dilution method, does not require pre-treatment of the soil with ¹⁵N enriched material.